The edge of kānuka shrubland that has been invaded by weeds

Mānuka and kānuka shrublands are spread across Aotearoa New Zealand and provide important economic, environmental, and cultural benefits. They are a source of high-value honey and traditional medicines, and they provide a refuge for native birds and a nursing ground for native plant regeneration.

Unfortunately, mānuka and kānuka shrublands are also under threat from many factors, including incursions by weeds. To better protect shrublands across the country, postgraduate fellow Dr. Laureline Rossignaud is conducting research that will help us understand what factors help mānuka and kānuka shrublands resist weed invasion, and to better understand the dynamics of weed invasions through time.

This research is part of Nuku-a-rangi, Pou 3 for Bioprotection Aotearoa.  Laureline’s project 3.2 is led by Dist. Prof. Philip Hulme which aims to determine the role of landscape attributes, ecosystem characteristics, and species traits on weed incursion in mānuka and kānuka shrublands. Laureline is using large datasets to investigate the drivers for weed invasions at the landscape scale.

The main database, hosted by Manaaki Whenua Landcare Research, is part of the national vegetation survey databank which provides plant information from permanent plots in mānuka and kānuka shrublands.

The drivers that Laureline is investigating include:

• the proportion of exotic lands surrounding the plots, which can host weeds and facilitate their incursion in the shrublands
• the distance to the nearest roads and rivers
• The climatic conditions at the plot (mean annual rainfall and temperature)
• the plant community already in the plot, including the structure of the vegetation with native species richness and native plant cover across different heights.

Laureline’s research so far has found that native tree species richness is very important in helping shrublands resist weed invasions because exotic species must compete with native trees for resources. In addition, most exotic weed species are adapted to open habitats, so tall native trees also make the ecosystem resilient against invasion by limiting access to light and space.

On the other hand, Laureline’s research also confirms that factors that help weeds invade shrublands include the proximity of exotic grasslands and the fragmentation of habitats.

Laureline is now creating maps to investigate the relationship between climate and weed invasions. These maps show that shrublands on the West Coast and in the southern South Island have fewer weed invasions than shrublands in other parts of the country. This is probably because these areas are largely in national parks and they are wetter and colder than other parts of the country. Climate is significantly correlated with weed invasion in the models, and warmer climates generally help exotic species establish.

Laureline says, “I have really enjoyed making maps and looking at the distribution of exotic species. When I was building the maps, I discovered some exotic species are spreading and others aren’t, which is quite interesting. I’m now having a more detailed look at why this is happening.”

Laureline’s research over the next year and a half will work to identify which weed species are extending their ranges and invading more plots, what factors are driving this spread, and what threats the weeds pose to mānuka and kānuka shrublands.

Laureline is enjoying being part of Bioprotection Aotearoa and says, “It’s a great opportunity because such a large number of scientists are working in this space. It’s a huge network and I’m going to learn a lot from all of the other cool projects. I feel excited and lucky.”